I. Field
The present disclosure relates generally to circuits, and more specifically to techniques for achieving linear monotonic output power from a transmitter.
II. Background
The design of a high performance transmitter is challenging due to various design considerations. First, many applications require high performance, which may be quantified by good linearity, high efficiency, etc. Second, low power consumption and low cost are highly desirable for applications such as wireless communication. In general, high performance and low power/cost impose conflicting design constraints.
In addition to the above design goals, some applications require a transmitter to have a wide range of output power. For example, a transmitter in a Code Division Multiple Access (CDMA) communication system may be required to adjust its output power over a range of approximately 85 decibels (dB) in steps of 1.0±0.5 dB. This wide output power range and small step size are used for closed loop power control. Since each transmitter causes interference to other transmitters in the CDMA system, the closed loop power control adjusts the output power of each transmitter to (1) achieve the desired received signal quality for that transmitter at a receiving base station and (2) minimize interference to other transmitters transmitting to the same base station.
To achieve the wide output power range with the required step resolution, a transmitter may be designed to be piecewise linear and monotonic. A function ƒ(x) is piecewise linear over a range of x values if this range can be partitioned into a finite number of subranges such that for each subrange function ƒ(x) can be represented by a linear function y=m×x+b, where m is a slope, b is an intercept point, and y is an output value. m and b may be different for different subranges. Function ƒ(x) is monotonic if it does not decrease as x increases. The entire output power range may be partitioned into smaller subranges in order to simplify the design of the transmitter. The transmitter is then designed such that the output power is linear and monotonic within each subrange as well as when switching from one subrange to another subrange. Using the function above, x may correspond to a gain control value, and ƒ(x) may correspond to the output power of the transmitter. The transmitter may utilize more complicated circuits, consume more battery power, and/or incur higher cost in order to achieve piecewise linear and monotonic characteristics.
There is therefore a need in the art for techniques to achieve linear monotonic output power even for a transmitter that is not piecewise linear and monotonic.